Connection of on an add/drop node

ABSTRACT

An add/drop node of an optical WDN-network which has two fiber paths for light of a plurality of channels propagating in opposite directions comprises two add/drop modules ( 231, 23   r ) for each of the channels. All the modules are identically constructed. Each module comprises an add device ( 251, 25   r ) for adding light to one of the paths and a drop device ( 27   r,    271 ) for deflecting a portion of light from a second one of the paths. A module comprises a house ( 41 ) enclosing the add device and the drop device. A first fixed connector ( 53, 73 ) is attached to the house for connection in the first path and to a an optical fiber ( 45, 65 ) which extends freely from the house and has a first free connector ( 43, 63 ) at its free end to be attached to the fixed connector of a neighboring add/drop module for continuing the first path through the considered add/drop module to the neighboring module. In the same way a second fixed connector is attached to the house for connection in the second path and to a second optical fiber which extends freely from the house and has a second free connector at its free end to be attached to the fixed second connector of a neighboring add/drop module for continuing the second path through the considered add/drop module to the neighboring module.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/SE00/01877 which has an Internationalfiling date of Sep. 27, 2000, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to an add/drop node of an opticalWDM-network, in particular to the connection of an add/drop node to twopaths of the network carrying light in opposite directions, and to anetwork including such an add/drop node.

BACKGROUND

Optical networks using WDM (Wavelength Division Multiplexing) are nowproposed to be built more and more. In such networks a plurality ofadd/drop nodes are connected and simple devices should be provided forconnecting the nodes to the network. For example, when an existing nodeis expanded to be capable of receiving and transmitting in anotherwavelength band the manual work required therefor should be minimized.An optical network having add/drop nodes is for example disclosed inU.S. Pat. No. 5,754,545.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an add/drop node for anoptical WDM-network having a simple way of connecting the node tocirculating fiber paths of the network.

It is another object of the invention to provide an optical WDM-networkhaving an add/drop node built to allow a simple way of connecting thenode to parallel fiber paths of the network.

Thus generally, an add/drop node is intended to be connected in anoptical WDM-network. The network has two parallel fiber paths allowinglight of a plurality of wavelength channels to propagate in oppositedirections. The add/drop node comprises two add/drop modules for each ofthe channels. All the modules are identically constructed and most oftheir connections are very similar to each other allowing a simplemounting and connection of the components of the node and also a simplerearrangement for changing wavelength channels and for adding/deletingwavelength channels used in the network. Each module comprises an adddevice for adding light to a first one of the paths and a drop devicefor deflecting a portion of light from a second one of the paths. Theadd device and the drop device of a module are enclosed by a housing,the housing of the modules being placed in a single row, at the sides ofeach other and for instance mounted in a rack. A first fixed connectoris attached to the housing of a module for connection in the first pathand to an optical fiber which extends freely from the housing and has afirst free connector at its free end to be attached to the fixedconnector of a neighbouring add/drop module for continuing the firstpath through the considered add/drop module to the neighbouring module.In the same way a second fixed connector is attached to the housing forconnection in the second path and to a second optical fiber whichextends freely from the house and has a second free connector at itsfree end to be attached to the fixed second connector of a neighbouringadd/drop module for continuing the second path through the consideredadd/drop module to the neighbouring module.

More particularly, in the add/drop node two add/drop modules areprovided for each of the channels of light propagating in the network.Each add/drop module has an add device or light combiner for addinglight to one of the two optical fiber paths and it has also a dropdevice for deflecting a portion of light from the other optical fiberpath. Furthermore, all the add/drop modules have the same construction.The add/drop modules can then be arranged in two sets, so that theadd/drop modules of a first set have their add devices connected in onefiber path and their drop devices connected in the other fiber path.Also, the add/drop modules of the second set then have their add devicesconnected in the other fiber path and their drop devices connected insaid one fiber path.

In at least each of the two sets the add/drop modules are placed at thesides of each other and then inner modules and two end modules areobtained in each set. For two adjacent or neighbouring add/drop modulesof a set an output of the add device in a first one of the two adjacentadd/drop modules can be connected to an input of the add device in asecond one of the two adjacent add/drop modules. In the same way, fortwo adjacent add/drop modules an output of the drop device in a firstone of the two adjacent add/drop modules can be connected to an input ofthe drop device in a second one of the two adjacent add/drop modules.For one of the end add/drop modules comprised in a first set its dropdevice can have an output connected to an input of the add device of oneof the two end add/drop modules in the other, second set. Similarly, forsaid one end add/drop module of the second set its drop device can havean output connected to an input of the add device of said one endadd/drop module in the first set.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of a non-limiting embodimentwith reference to the accompanying drawings, in which

FIG. 1 is a block diagram of an optical network having a ringarchitecture and a hub node and four client nodes,

FIG. 2 is a schematic picture of the connection lines of an add/dropmodule used in the nodes of the network,

FIG. 3 is a view from the side of the inside of a housing of an add/dropmodule,

FIG. 4 is a front view of a plurality of add/drop modules mounted in arack,

FIG. 5 is a block diagram schematically showing the connections ofadd/drop modules of the nodes,

FIG. 6 is a block diagram showing in a somewhat more realistic way theconnections of add/drop modules of the nodes, and

FIG. 7 is a perspective view showing the inside of a housing of anadd/drop module.

DETAILED DESCRIPTION

In FIG. 1 a block diagram of an optical fiber WDM-network having a ringconfiguration is shown. Thus two optical fiber paths 1 e, 1 w pass in abasically uninterrupted way all around the network, one fiber path 1 ecarrying light propagating in the east direction and the other fiberpath 1 w carrying light propagating in the west direction.

The network includes a hub node 2 and in the embodiment shown fourclient nodes 3, called Client 1, 2, 3 and 4, the nodes being connectedto the two basic fiber paths 1 e, 1 w for adding and dropping light fromthe fibers. A client node n receives and transmits information in anarrow wavelength band, also called channel, around a single wavelengthλ_(n), n=1, 2, . . . . The hub node 2 can receive and transmitinformation in all channels, i.e., on all wavelengths λ_(n), n=1, 2, . .. . For each client node 3, the hub node 2 is connected to an electricalclient portion 5. Such an electrical client portion 5 comprises anelectrooptic converter or optical transmitter 7 converting electricalsignals to optical signals and an optoelectric converter or opticalreceiver 9 for receiving optical signals converting the received signalsto electrical signals. The electrical client portion 5 is throughoptical fibers connected to an optical client portion 11 in the hub node2. The optical client portion 11 has optical connectors for receivingthe optical fibers extending from the respective electrical clientportion.

The optical client portion 11 comprises an optical receiver-transmittercombination 13, 15 for transmission, the receiver 13 of the combinationreceiving the light signal from the transmitter 7 of the electricalclient portion 5 providing its output signal to the transmitter 15 ofthe pair, which provides a well-defined light signal in the narrowwavelength band used for the respective client. The optical transmitter15 is coupled to an optical connector for providing its output signal onan optical fiber to add/drop modules as will be described hereinafter.

The optical client portion 11 also comprises an opticalreceiver-transmitter combination for receiving, the combinationcomprising two optical receivers 17 e, 17 w connected to receive lightfrom the add/drop modules through optical fibers and optical connectors,one receiver 17 e being used for receiving light propagating in thenetwork, in the appropriate fiber, in an east direction and anotherreceiver 17 w being used for receiving light propagating in the westdirection in the ring network. The outputs of the two optical receivers17 e, 17 w are connected to inputs of a combining element or opticalmultiplexer 19 which combines the received signals to provide them to atransmitter 21, the output terminal of which is through the respectiveconnector and a fiber length connected to the receiver 9 in theelectrical client portion 5.

Furthermore the hub node 2 includes a plurality of add/drop modules 231,23 r one pair of such add/drop modules being provided for each clientnode 3 in the network. In such a pair one module 231 is adapted totransmit in a left direction from the hub node and to receive from thesame left direction. The other module 23 r of a pair is adapted totransmit in and to receive from the right direction from the hub node.Each add/drop module is connected in the two ring-shaped fiber paths 1e, 1 w of the network. The left add/drop module 231 comprises an adddevice 251 connected in the fiber ring path 1 w and an drop device 271connected in the other fiber ring path 1 e. The add device 251 is,through fiber sections, a 50/50 splitting coupler 29 (only one is shownin the drawing) and the respective connector connected to thetransmitter 15 in the optical client portion 11 for the appropriateclient node. The drop device 271, is through a fiber and the respectiveconnector, connected to the receiver 17 e in the optical client portion11 for the same client node.

In the same way, the right add/drop module 23 r in the pair comprises anadd device 25 r connected in the fiber ring path 1 e and an drop device27 r connected in the other fiber ring path 1 w. The add device 25 r is,through fiber sections, a respective splitting coupler 29 and therespective connector, connected to the transmitter 15 in the opticalclient portion 11 for the client node. The drop device 27 r is, througha fiber and the respective connector, connected to the receiver 17 w inthe optical client portion 11 for the client node.

The add devices 251, 25 r contain some coupling or combining elementand, if required, a notch filter blocking light of the wavelength bandor channel for which the add/drop module is designed. The optionalfilter will then stop only light of the wavelength band propagating inthe respective fiber ring path 1 w, 1 e before light of the samewavelength band is added in the combining element. The drop devices 271,27 r contain in the same way some splitting and filtering element fortapping off only light of the wavelength band or channel for which theadd/drop module is designed.

All add/drop nodes 231, 23 r have the same basic design and functionsand can thus all be given the same physical shape as discussedhereinafter.

The two fiber ring paths 1 e, 1 w are connected to the hub node 2 on aleft side of the hub node and on a right side of the node. On each suchside a monitor module 311, 31 r can be arranged which is thus connectedin the two ring paths. The monitor module 311, 31 r comprises an addcoupler 331, 33 r for adding, e.g., some control signal and a tap 351,35 r for tapping off some small portion of the incoming light power(e.g., 1%.).

The schematic diagram of FIG. 2 illustrates the connections of theadd/drop modules 231, 23 r. Each module comprises a housing indicated at41. Light from one (1 e) of two ring paths of the network enters themodule at a connector 43 attached to a fiber section 45 extendingloosely outside the housing 41. The fiber section 45 has a thickprotective sleeve which extends inside the housing to a weld section 47which splices an end of an optical fiber 49 having a standard thinprotective sleeve to the fiber section 45. The optical fiber section 49is, at its opposite ends connected to one of the two inputs of the adddevice 251, 25 r. The output of the add device is connected to a fibersection 51, which in turn is connected to a connector 53 attached to thehousing 41. The connector 53 should be connected in the same ring path 1e as the input connector 43. The other input of the add device is,through a fiber section 55, connected to a connector 57 which isattached to the housing 41. The connector 43 thus receives light fromthe ring path 1 e to the fiber 45, through the weld 47, the fibersection 49, the add device 251, 25 r, the fiber section 51 and to theconnector 53. The output connector 53 lets the light continue along thering path 1 e. Light from an optical client portion 11 enters the moduleat the connector 57, continues through the fiber 55 to the add device251, 25 r, in which the light is added to that propagating along thering path 1 e.

Furthermore, in the module 231, 23 r there is a set of connected fibersections, which is parallel to that described and is connected in theother ring path 1 w of the network, but in which instead of the adddevice 25 r, 251, the drop device 271, 27 r is connected. Thus aconnector 63 is to be connected to the other ring path and is attachedto an end of a well-protected fiber section 65 extending partly outsidethe housing 41; the other end being the fiber section 65 connectedthrough a weld section 67 to a standard fiber section piece 69. Thisfiber section is in turn connected to one of the two outputs of the dropdevice 271, 27 r. The input of the drop device is connected to a fibersection 71 which receives light from a connector 73 attached to thehousing 41. The connector 73 should be connected in the respective ringpath 1 w. The other output of the drop device 271, 27 r is connected toan end of a fiber section 75 which has its other end connected to aconnector 77 attached to the housing 41. This connector 77 is, through afiber, attached to a respective receiver 17 w (17 e) in the opticalclient portion 11. Light from the ring path 1 w enters the module at theconnector 73, continues through the fiber 71, the drop device 271, 27 r,the fiber 69, the weld section 67, the thick fiber 65 to the looseconnector 63, which in turn is connected in the respective ring path 1 wof the network. Some light of a specific wavelength band is tapped offin the drop device 271, 27 r and continues through the fiber 75 to theconnector 77 and therefrom to the respective optical client portion 11.

The physical layout of the interior of an add/drop module housing 41 isshown in the side view of FIG. 3. The module housing 41 comprises asubstantially flat portion from which various walls stand out. The wallsall have the same height and connect to a basically flat lid (not shown)which is mounted over the housing 41. The walls form two circularwinding cores 81 having a sufficiently large diameter, e.g. about 50 mm,allowing that fibers can be wound around them and not being subjected totoo small bending radii (too severe curvatures). The two winding cores81 are placed at some distance from each other to allow the fibers topass therebetween. By arranging two such cores the fiber sections usedcan be allowed to have some extra length allowing them to be comfortablyhandled and to again be spliced to the devices in the case of fiberbreaks or bad splices. Additionally, the direction of the fiber sectionsat the places where they are connected to the devices can be selected toavoid bends which are too small, by placing the fibers, for instance, ina configuration similar to the figure eight around the two cores 81.Inside the walls forming the winding cores through-holes 83 may bearranged for an easy handling of the module housing. The add devices251, 25 r and the drop devices 271, 27 r can be attached betweenoutstanding walls 85 at the top of the housing.

At the lower edge of the housing 41 devices are provided for attachingthe housing to a rack, the attaching devices comprising a notch 87 atthe rear side and a snap device 89 at the front side. A channel 91 isformed at the front top side of the house 41 to allow fibers connectingthe module to the associated optical client portion to be held therein.

FIG. 4 is a front view of the add/drop modules 231, 23 r and the monitormodules 311, 31 r mounted in a rack, the loosely extending fiber pieces45, 65 not being visible in this figure. By comparing FIG. 4 to FIG. 1it is seen that the connection of all left add/drop modules 231 is asindicated in FIG. 3, the extending fiber sections 45, 65 being insertedin the mating connectors 53, 73 in the adjacent module at the left sideof the respective module. This connection is illustrated in theschematic view of FIG. 6; see also FIG. 5. Thus, in FIG. 5 the samebasic connection as in FIG. 1 is illustrated, where, in the rightmodules 23 r the add devices and the drop devices have changed placeswith each other. Then a cross coupling must be made between the group ofleft modules 231 and the group of right modules 23 r and between theright group and the right monitor module 31 r as compared to thestraight schematic connection of modules shown in FIG. 1.

The same connection of the modules is illustrated in FIG. 6 in which theconnections between the add/drop modules using loosely extending fibersections 45, 65 is shown. From this figure it appears clearly that alladd/drop modules 231, 23 r can have an identical construction, only thetapping-off devices and filters being specific to the channel for whichthe respective module is designed. The cross coupling is made in themiddle of the assembly of add/drop modules, between the left and rightmodules. Such a cross connection can be made by connecting the loosefiber sections 101 having optical connectors at each end in a crossconfiguration or a particular cross connecting module can be used. Sucha module has the same exterior design as the other modules but hasinside just the optical fibers connected cross-wise.

In FIG. 4 a multitude of add/drop modules 231, 23 r are illustrated.However, only a portion of the add/drop modules may be active ones,constructed as described above. At the side of the active modules dummymodules are inserted having the same exterior layout but without theconnectors and the loosely extending fibers. The dummy modules are usedfor just filling up the space between the active modules and the monitormodules.

The two monitor modules 311, 31 r both have the same exterior connectorsand interior devices as each other but have not the same interiorconnection lines. The necessary connections appear clearly from FIG. 1.

The client nodes 3 in the network have the same basic design as the hubnode 2 but are designed to receive and transmit in only one wavelengthband. The same kind of add/drop modules as described above can forexample be used.

In other network ring architectures the client nodes can receive andtransmit in more than one wavelength band. Then the client nodes canhave the same structure as the hub node 2.

1. An add/drop node to be connected in an optical WDM-network, thenetwork including two optical fiber paths for letting light of aplurality of channels propagate in opposite directions in the network,characterized by two add/drop modules for each of the channels, eachadd/drop module comprising an add device for adding light to a first oneof the two optical fiber paths and a drop device for deflecting aportion of light from a second one of the two optical fiber pathsdifferent from the first one and all add/drop modules having the sameconstruction, and in that the add/drop modules are arranged in two sets,the add/drop modules of a first one of the two sets having their adddevices connected in the first one of the two optical fiber paths andtheir drop devices connected in the second one of the two optical fiberpaths and the add/drop modules of a second one of the two sets differentfrom the first one having their add devices connected in the second oneof the two optical fiber paths and their drop devices connected in thefirst one of the two optical fiber paths.
 2. An add/drop node accordingto claim 1, characterized in that in each of the two sets the add/dropmodules are placed at the sides of each other, and that for two adjacentadd/drop modules an output of the add device in a first one of the twoadjacent add/drop modules is connected to an input of the add device ina second one of the two adjacent add/drop modules.
 3. An add/drop nodeaccording to claim 2, characterized in that in each of the two sets theadd/drop modules are placed at the sides of each other to form inneradd/drop modules and two end add/drop modules in each set, the endadd/drop modules having an add/drop module of the set on only one side,and that for one of the two end add/drop modules of a first one of thetwo sets its drop device has an output connected to an input of the adddevice of one of the two end add/drop modules in a second one of the twosets and that for said one of the two end add/drop modules of the secondone of the two sets its drop device has an output connected to an inputof the add device of said one of the two end add/drop modules in thefirst one of the two sets.
 4. An add/drop node according to claim 2,characterized in that each add/drop module comprises a housing enclosingthe add device and the drop device of the add/drop module, a first fixedconnector attached to the housing for connection in the first one of thetwo optical fiber paths and a first optical fiber extending freely fromthe housing and having a first free connector at its free end to beattached to the fixed connector of a neighboring add/drop module forcontinuing the first path through the considered add/drop module to theneighboring module, and a second fixed connector attached to the housingfor connection in the second one of the two optical fiber paths and asecond optical fiber extending freely from the housing and having asecond free connector at its free end to be attached to the fixed secondconnector of a neighboring add/drop module for continuing the secondpath through the considered add/drop module to the neighboring module.5. An add/drop node according to claim 1, characterized in that in eachof the two sets the add/drop modules are placed at the sides of eachother, and that for two adjacent add/drop modules an output of the dropdevice in a first one of the two adjacent add/drop modules is connectedto an input of the drop device in a second one of the two adjacentadd/drop modules.
 6. An add/drop node according to claim 5,characterized in that in each of the two sets the add/drop modules areplaced at the sides of each other to form inner add/drop modules and twoend add/drop modules in each set, the end add/drop modules having anadd/drop module of the set on only one side, and that for one of the twoend add/drop modules of a first one of the two sets its drop device hasan output connected to an input of the add device of one of the two endadd/drop modules in a second one of the two sets and that for said oneof the two end add/drop modules of the second one of the two sets itsdrop device has an output connected to an input of the add device ofsaid one of the two end add/drop modules in the first one of the twosets.
 7. An add/drop node according to claim 5, characterized in thateach add/drop module comprises a housing enclosing the add device andthe drop device of the add/drop module, a first fixed connector attachedto the housing for connection in the first one of the two optical fiberpaths and a first optical fiber extending freely from the housing andhaving a first free connector at its free end to be attached to thefixed connector of a neighboring add/drop module for containing thefirst path through the considered add/drop module to the neighboringmodule, and a second fixed connector attached to the housing forconnection in the second one of the two optical fiber paths and a secondoptical fiber extending freely from the housing and having a second freeconnector at its free end to be attached to the fixed second connectorof a neighboring add/drop module for continuing the second path throughthe considered add/drop module to the neighboring module.
 8. An add/dropnode according to claim 1, characterized in that in each of the two setsthe add/drop modules are placed at the sides of each other to form inneradd/drop modules and two end add/drop modules in each set, the endadd/drop modules having an add/drop module of the set on only one side,and that for one of the two end add/drop modules of a first one of thetwo sets its drop device has an output connected to an input of the adddevice of one of the two end add/drop modules in a second one of the twosets and that for said one of the two end add/drop modules of the secondone of the two sets its drop device has an output connected to an inputof the add device of said one of the two end add/drop modules in thefirst one of the two sets.
 9. An add/drop node according to claim 8,characterized in that each add/drop module comprises a housing enclosingthe add device and the drop device of the add/drop module, a first fixedconnector attached to the housing for connection in the first one of thetwo optical fiber paths and a first optical fiber extending freely fromthe housing and having a first free connector at its free end to beattached to the fixed connector of a neighboring add/drop module forcontinuing the first path through the considered add/drop module to theneighboring module, and a second fixed connector attached to the housingfor connection in the second one of the two optical fiber paths and asecond optical fiber extending freely from the housing and having asecond free connector at its free end to be attached to the fixed secondconnector of a neighboring add/drop module for continuing the secondpath through the considered add/drop module to the neighboring module.10. An add/drop node according to claim 1, characterized in that eachadd/drop module comprises a housing enclosing the add device and thedrop device of the add/drop module, a first fixed connector attached tothe housing for connection in the first one of the two optical fiberpaths and a first optical fiber extending freely from the housing andhaving a first free connector at its free end to be attached to thefixed connector of a neighboring add/drop module for continuing thefirst path through the considered add/drop module to the neighboringmodule, and a second fixed connector attached to the housing forconnection in the second one of the two optical fiber paths and a secondoptical fiber extending freely from the housing and having a second freeconnector at its free end to be attached to the fixed second connectorof a neighboring add/drop module for continuing the second path throughthe considered add/drop module to the neighboring module.
 11. Anadd/drop node according to claim 10, characterized in that the houseincludes two winding cores around which excessive fiber lengthsconnecting devices and connectors of the add/drop module can be wound.